In the fabrication of integrated circuits and other electronic devices, multiple layers of conducting, semiconducting, and dielectric materials are deposited on or removed from a surface of a semiconductor wafer. Thin layers of conducting, semiconducting, and dielectric materials may be deposited by a number of deposition techniques. Common deposition techniques in modern processing include physical vapor deposition (PVD), also known as sputtering, chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD), and electrochemical plating (ECP).
As layers of materials are sequentially deposited and removed, the uppermost surface of the substrate may become non-planar across its surface and require planarization. Planarizing a surface, or “polishing” a surface, is a process where material is removed from the surface of the wafer to form a generally even, planar surface. Planarization is useful in removing undesired surface topography and surface defects, such as rough surfaces, agglomerated materials, crystal lattice damage, scratches, and contaminated layers or materials. Planarization is also useful in forming features on a substrate by removing excess deposited material used to fill the features and to provide an even surface for subsequent levels of metallization and processing.
Chemical mechanical planarization, or chemical mechanical polishing (CMP), is a common technique used to planarize substrates such as semiconductor wafers. In conventional CMP, a wafer carrier or polishing head is mounted on a carrier assembly and positioned in contact with a polishing pad in a CMP apparatus. The carrier assembly provides a controllable pressure to the substrate urging the wafer against the polishing pad. The pad is moved (e.g., rotated) relative to the substrate by an external driving force. Simultaneously therewith, a chemical composition (“slurry”) or other fluid medium is flowed onto the substrate and between the wafer and the polishing pad. The wafer surface is thus polished by the chemical and mechanical action of the pad surface and slurry in a manner that selectively removes material from the substrate surface.
A problem encountered when planarizing a wafer is knowing when to terminate the process. To this end, a variety of planarization end-point detection schemes have been developed. One such scheme involves optical in-situ measurements of the wafer surface. The optical technique involves providing the polishing pad with a window transparent to select wavelengths of light. A light beam is directed through the window to the wafer surface, where it reflects and passes back through the window to a detector, e.g., an interferometer. Based on the return signal, properties of the wafer surface, e.g., the thickness of films (e.g., oxide layers) thereon, can be determined.
While many types of materials for polishing pad windows can be used, in practice the windows are typically made of the same material as the polishing pad, e.g., polyurethane. For example, U.S. Pat. No. 6,280,290 discloses a polishing pad having a window in the form of a polyurethane plug. The pad has an aperture and the window is held in the aperture with adhesives.
A problem with such windows arises when they have surface roughness. For example, polyurethane windows are typically formed by slicing a section from a polyurethane block. Unfortunately, the slicing process produces surface imperfections or roughness R on either side of the window 1 in polishing pad 10, as shown in FIG. 1. The depth of the roughness ranges from about 10 to about 100 microns. The roughness on the bottom surface scatter the light used to measure the wafer surface topography, thereby reducing the signal strength of the in-situ optical measurement system. The roughness on the upper surface do not tend to scatter light as much as the bottom surface roughness due to the presence of a liquid slurry and proximity of the upper surface to the wafer.
Because of the loss in signal strength from scattering by the lower window surface, the measurement resolution suffers, and measurement variability is a problem. Accordingly, what is needed a polishing pad for chemical-mechanical planarization with an improved window having greater light transmission and less light scattering properties.